Provide option to allow --secret argument be interpreted as base-64 encoded bytes

[ceharris]

Summary

To simplify use in scripted applications, it would be helpful if there was an option to interpret the --secret argument as a string containing base-64 encoded bytes. Perhaps an additional --base64 or -B option could be used to specify this behavior.

The same outcome can be achieved by writing the bytes of the key to a file and using --secret @filename, but directly supporting base-64 would avoid the need to write the key to a file.

Example

jwt --base64 --secret "CFP7qWsQP/oHNj+i+mxpEf/iYzmlweFHwonE4nlUCtA="

[ceharris]

See PR #144